In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each cellular telephone. To place a phone call, the individual subscriber or other authorized user of a cellular telephone dials a telephone number and presses the "Send" button. In response, the cellular telephone transmits its ESN and MIN to the cellular system so the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by "cloning" the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular system without paying for the service. A pirate can obtain the ESN and MIN of a cellular telephone by monitoring a transmission of the cellular telephone, and then programming them into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN are inadequate to protect a cellular telephone system from illegal use by pirates.
In an effort to provide additional security, some cellular systems authenticate cellular telephones based on the transmission of data by the cellular telephone during a call set-up process. Rather than identify the cellular telephone by its ESN and MIN alone, the system identifies a cellular telephone by its transmission characteristics. In this manner, the cellular system can reject calls from cloned cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), incorporated herein by reference, a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient reference waveform is used to identify the type of transmitter. The transmission characteristics of the reference waveform can be processed in different manners to create a "fingerprint" of the individual transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other fingerprint characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 to Rudokas on May 30, 1995 ("the Rudokas patent"), incorporated herein by reference, describes an identifier, such as a radio frequency signature, that can be used to positively identify a valid cellular telephone or a known fraudulent telephone. Other types of signature or fingerprint authentication systems are also known in the art.
Fingerprint authentication systems all require at least one transmission characteristic waveform, known to be generated by the authentic cellular telephone, to be used as a reference waveform for the fingerprint authentication system. Some systems may rely on more than one reference waveforms to generate the fingerprint. The term fingerprint as used herein is intended to include reference waveforms transmitted by a transmitter as well as the various transmission characteristics derived from those reference waveforms.
These fingerprint authentication systems require a substantial amount of data processing time to establish the fingerprint. However, this data processing is not time dependent and may be performed over a period of time. In contrast, the process of comparing the transmission characteristic of the unauthenticated transmitter with the stored fingerprint must be performed in real-time and quickly to effectively deny or terminate fraudulent calls. This can place difficult demands on a fingerprint authentication system's processing capabilities during peak calling periods. Some existing cellular telephone systems use non-fingerprint analysis techniques that are not real time. In those systems, if a call is subsequently determined to be fraudulent, the destination telephone number is added to a list of known fraudulent numbers. The cellular telephone system checks the list of known fraudulent numbers to prevent any subsequent telephone calls from being placed to a known fraudulent number.
Another drawback of these fingerprint authentication systems is that it is possible that a fraudulent cellular telephone will be misidentified as an authorized cellular telephone because no fingerprint analysis system is 100% effective. Conversely, it is possible that an authorized cellular telephone will be misidentified as fraudulent. While the former misidentification may result in incorrect charges to the customer, and the resulting loss of revenues to the service provider, the latter misidentification is of particular concern to service providers. Specifically, service providers do not wish to inconvenience the customer and disrupt valid calls due to the misidentification of an authorized cellular telephone as fraudulent or invalid.
Therefore, it can be appreciated that there is a significant need for a system and method for detecting fraudulent calls using characteristics other than a fingerprint waveform analysis thus preventing overloading of a cellular telephone fingerprint system's processing capabilities. There is also a significant need to reduce misidentification of valid calls as fraudulent by the fingerprint analysis method. The present invention provides these and other advantages as will be illustrated by the following description and accompanying figures.